Lenses – How to Choose a lens, and what to look for – Autofocus Types, Manual Focus, and Which Specifications for What?

In this article I will give you some tips on what to look for in a lens…

Compatibility

You can’t use a lens if it’s not compatible with your camera! Always ensure that a lens is compatible with your camera before you even think about buying it!

If you have a Sony camera, simply buying any random Sony lens is not enough, you need to ensure that the mount is correct (A-mount camera can take A-mount lenses only). Also, if you have an SLT camera, double check for compatibility, as there is not a large range of SLT compatible lenses.

Function

Firstly, what is the lens for? If you need it for landscapes, it is worth looking at a wide-angle lens with a narrow minimum aperture (such as f/22 or f/32). The maximum aperture is not critical for a landscape photographers lens, as you will be generally using narrower apertures, but if possible look for a lens with a maximum aperture that is no smaller than f/5.6 or f/6.3.

If the lens is for portraits, you will need a fast lens with a maximum aperture of f/1.4 or f/2.8, and no smaller than 3.5 to ensure that you can achieve delicious, smooth blur. Also, try to aim for a 100mm portrait lens as opposed to a 35 of 50mm one, as these can distort the perspective slightly, which may result in big noses, etc. Lenses with a larger focal length look better, because they compress the perspective, meaning the whole face is more likely to be in focus, and big noses are less of a problem.

If the lens is for wildlife photography, you need to be careful to avoid getting caught out.

Wildlife is usually flighty, so you need a long lens to avoid scaring the animals by getting too close. So, look for a lens with a long focal length. Something like 75-350mm is usually sufficient for snap-happy amateurs, but if you want to do more extreme wildlife photography, a 300-600mm may be more like what you require.

Wildlife is also mostly fast and nimble, so you need a fast shutter speed. To be able to use a fast shutter speed, a large aperture is very helpful. A lens with a maximum aperture of f/7.1 will be much darker than a f/3.5 lens, so one with a large aperture will be much more expensive. Also, technology does not seem to make them any brighter, f/2.8 telephoto lenses are hard to come by, you are much more likely to encounter a f/5.6 or darker lens.

Also, due to animals speed, good autofocus is also important. Preferably, look for a lens with a ring-type ultrasonic focus motor, as this is fast, and almost silent.

Autofocus Types

The most basic lenses of all have no focus motor, and need to be driven by a screw drive from the camera. If the camera does not have this, autofocus won’t work at all!

The next step us is an electric motor, which allows the lens to autofocus on it’s own. This is very slow, and often has difficulty locking onto the subject. These lenses often do not have a manual focus option, and autofocus is also very noisy – so noisy any wildlife would probably run away! The lens also extends to focus, often as much as 1.5 to 2 inches!

An ultrasonic motor is a slight improvement over this. It operates at a frequency that is not audible to humans (usually 30kHz), however, it still requires gear wheels that make a little noise. It is also no faster than an electric motor.

The best type is a ring-type ultrasonic. These don’t use a motor, instead they use two large rings inside the lenses barrel that drives autofocus quickly, accurately, with virtually no noise, and no lens extension. These lenses also always have a manual focus option.

Features

Manual Focus is a very useful feature for a lens to have – it allows you to override the autofocus suggestions, meaning you can select where you want the lens to focus. For macro photography, this is essential, as without it, you cannot obtain the accurate results you want. It is also very useful for fine art photography, and sports photography, as you can preset manual focus, and when the subject enter the frame, you can take a shot. This helps counter focus inconsistency.

Elements, Blades and Groups???? What The…?

A Lens Element is just a single piece of glass in the lens. A lens has multiple of these, some are ordinary ‘spherical ‘ elements, some are ‘aspherical’, and others are constructed from a special type of glass. In general, more elements, and the use of special glass, or aspherical elements allows for less optical faults.

When two or more elements come in contact with each other – they form a group.

Lens ‘Blades‘ refer to the number of diaphragm blades that form the aperture. The diaphragm size is affected by the f/stop that you use. A lens will have 5 or more blades. If there were only 3, the aperture would be triangular, four would make it square. So, the more the better? No, not necessarily, some lenses have less, than others, but still produce better quality. Between 6 and 11 blades is usually the norm.